Ruthenium Oxidation Complexes

Catalysis by Metal CoMplexes

This book series covers topics of interest to a wide range of academic and industrial chemists, and biochemists. Catalysis by metal complexes plays a prominent role in many processes. Developments in analytical and synthetic techniques and instru-mentation, particularly over the last 30 years, have resulted in an increasingly sophisticated understanding of catalytic processes.

Industrial applications include the production of petrochemicals, fine chemicals and pharmaceuticals (particularly through asymmetric catalysis), hydrometallurgy, and waste-treatment processes. Many life processes are based on metallo-enzyme systems that catalyse redox and acid-base reactions.

Catalysis by metal complexes is an exciting, fast developing and challenging interdisciplinary topic which spans and embraces the three areas of catalysis: heterogeneous, homogeneous, and metallo-enzyme.

Catalysis by Metal Complexes deals with all aspects of catalysis which involve metal complexes and seeks to publish authoritative, state-of-the-art volumes which serve to document the progress being made in this interdisciplinary area of science.

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Brain R. James played a significant editorial role in the realization of this volume

VoluMe 34: RutheniuM oxidation CoMplexes

Volume Author

W.P. GriffithDepartment of Chemistry

Imperial CollegeLondon, SW7 2AZUnited Kingdom

For other titles published in this series,go to http:// www.springer.com/series/5763

Prof. William P. Griffith

Ruthenium Oxidation Complexes

Their Uses as Homogenous Organic Catalysts

ISBN 978-1-4020-9376-0 e-ISBN 978-1-4020-9378-4DOI 10.1007/978-1-4020-9378-4Springer Dordrecht New York Heidelberg London

Library of Congress Control Number: 2010935300

© Springer Science+Business Media B.V. 2011No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission from the Publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work.

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Springer is part of Springer Science+Business Media (www.springer.com)

Prof. William P. GriffithDepartment of ChemistryImperial CollegeLondon, SW7 2AZUnited Kingdomw.griffith@imperial.ac.uk

v

This book is concerned with the application of ruthenium complexes as catalysts for useful organic oxidations.

Ruthenium was the last of the platinum-group elements to be discovered, and has perhaps the most interesting and challenging chemistry of the six. In this book just one major aspect is covered: its ability, mainly by virtue of its remarkably wide range of oxidation states which exist in its many complexes (from +8 to −2 inclu-sive) to effect useful and efficient oxidations of organic substrates.

Emphasis has been placed on useful organic oxidations, particularly on the cata-lytic production of natural products or pharmaceuticals and of the oxidation of important organic functional groups such as alcohols, alkenes, alkynes, amines and heteroatomic functionalities. The book is not directed solely at organic chemists. It is hoped that the inorganic, organometallic and coordination chemist will draw from it useful information, particularly from Chapter 1. The coverage of this very large subject can not claim to be entirely comprehensive, but it is hoped that all the main references, including some from 2010, have been covered.

The first chapter concerns the chemistry of the oxidation catalysts, some 250 of these, arranged in decreasing order of the metal oxidation state (VIII) to (0). Preparations, structural and spectroscopic characteristics are briefly described, fol-lowed by a summary of their catalytic oxidation properties for organic substrates, with a brief appendix on practical matters with four important oxidants. The subse-quent four chapters concentrate on oxidations of specific organic groups, first for alcohols, then alkenes, arenes, alkynes, alkanes, amines and other substrates with hetero atoms. Frequent cross-references between the five chapters are provided.

I would like to thank my good friend Brian James of the University of British Columbia (UBC), who suggested some years ago that I write a book of this type and who has carefully read and trenchantly criticised large sections of it. From Imperial College I am deeply indebted to Ed Smith who has read all of it and pointed out a number of errors and inconsistencies, and Ed Marshall who has drawn all the figures. I am very grateful to Steve Ley from Cambridge who has read the section on TPAP, the reagent which he, I and our co-workers developed. Any remaining errors and omissions are entirely my responsibility. All the cited refer-ences have of course been consulted and, though most are from online sources, it

preface

vi Preface

is a pleasure to thank the librarians of Imperial College, the Science Museum and the Royal Society of Chemistry libraries, and of course of the British Library. Finally, I am deeply grateful to my wife Anne (and in earlier years to my daughters Helen and Miranda) for her forbearance during the long time this book has taken to compile.

vii

Contents

1 the Chemistry of Ruthenium oxidation Complexes ........................... 1 1.1 Overview and Introduction ................................................................ 1 1.1.1 Discovery of Ruthenium ........................................................ 3 1.1.2 Oxidation States in Ruthenium Complexes ........................... 3 1.1.3 Reviews on Ruthenium Complexes

in Oxidation Reactions ........................................................... 4 1.1.4 Reviews on Oxidations of Organic Substrates

by Ru Complexes ................................................................... 4 1.1.5 Syntheses of Natural Products or Pharmaceuticals

by Ru Catalysts ...................................................................... 5 1.2 Ru(VIII) Complexes ........................................................................... 7 1.2.1 Ruthenium Tetroxide, RuO

4 ................................................... 7

1.2.2 Preparation ............................................................................. 7 1.2.3 Physical Properties ................................................................. 8 1.2.4 Analysis and Toxicity ............................................................. 10 1.2.5 RuO

4 as an Organic Oxidant .................................................. 11

1.2.6 Co-oxidants and Solvents for RuO4 Oxidations ..................... 12

1.2.7 Oxidations Effected by RuO4 ................................................. 14

1.2.8 Other Ru(VIII) Complexes..................................................... 30 1.3 Ru(VII) Complexes ............................................................................ 30 1.3.1 Perruthenates, TPAP and [RuO

4]− .......................................... 30

1.3.2 Preparation ............................................................................. 31 1.3.3 Physical Properties ................................................................. 32 1.3.4 TPAP and [RuO

4]− as Organic Oxidants ................................ 33

1.3.5 Other Ru(VII) Species............................................................ 40 1.4 Ru(VI) Complexes ............................................................................. 40 1.4.1 Ruthenates, [RuO

4]2− .............................................................. 41

1.4.2 Ru(VI) Complexes Containing Dioxo Moieties..................... 47 1.4.3 Ru(VI) Complexes Containing Mono-Oxo

or Nitride Donors ................................................................... 66

viii Contents

1.5 Ru(V) Complexes ............................................................................... 67 1.6 Ru(IV) Complexes ............................................................................. 69 1.6.1 Ruthenium Dioxide, RuO

2 and RuO

2.nH

2O ........................... 70

1.6.2 Ru(IV) Complexes with O- or N-Donors ............................... 71 1.7 Ru(III) Complexes.............................................................................. 76 1.7.1 Ru(III) Complexes with O-Donors ........................................ 76 1.7.2 Ruthenium Trichloride, RuCl

3 and RuCl

3.nH

2O .................... 79

1.7.3 RuBr3 and Halo-Aqua Complexes ......................................... 83

1.7.4 Ru(III) Complexes with Chelating O-, N-Donors .................. 83 1.7.5 Ru(III) Complexes with N-Donors ........................................ 85 1.7.6 Ru(III) Complexes with -P, -As, -Sb

and -S Donors ......................................................................... 88 1.8 Ru(II–III) Complexes ......................................................................... 89 1.9 Ru(II) Complexes ............................................................................... 89 1.9.1 Ru(II) Complexes with O- and N-Donors .............................. 90 1.9.2 Ru(II) Complexes with Porphyrin, Phthalocyanine

and Macrocyclic Donors ........................................................ 95 1.9.3 Ru(II) complexes with -P, -As and -Sb Donors ...................... 97 1.9.4 Ru(II) Complexes with -S and -O Donors ............................. 105 1.9.5 Ru(II) Complexes with -C Donors ......................................... 108 1.10 Ru(0) Complexes................................................................................ 110 1.11 Appendix: Brief Resumé of Preparations of Ru Oxidants

and Oxidation Reactions .................................................................... 110 1.11.1 Preparations of RuO

4 In Situ for Oxidations ........................ 110

1.11.2 Preparations and Use of [RuO4]− .......................................... 111

1.11.3 Preparations and Use of [RuO4]2− ......................................... 112

1.11.4 Preparation of trans-Ru(O)2(bpy)

IO3(OH)

31.5H

2O ............................................................... 113

References ......................................................................................................... 113

2 oxidation of alcohols, Carbohydrates and diols ................................... 135 2.1 Primary Alcohols to Aldehydes (Table 2.1) ....................................... 136 2.1.1 Model Substrates .................................................................... 137 2.1.2 Specific Examples .................................................................. 137 2.1.3 Natural Product/Pharmaceutical Syntheses

Involving Primary Alcohol Oxidations .................................. 139 2.2 Primary Alcohols to Carboxylic Acids (Table 2.1) ............................ 141 2.2.1 Model Substrates .................................................................... 141 2.3 Secondary Alcohols to Ketones or Lactones...................................... 142 2.3.1 Model Substrates .................................................................... 142 2.3.2 Specific Examples .................................................................. 145 2.3.3 Natural Product/Pharmaceutical Syntheses

Involving Secondary Alcohol Oxidations .............................. 146 2.3.4 Hydroxylactones, a-Hydroxy Esters and Cyanohydrins ....... 148 2.3.5 Deracemisation of Secondary Alcohols ................................. 148

ixContents

2.3.6 Alcohols, Carbohydrates and Diols Not Covered Here but Included in Chapter 1 .............................................. 149

2.3.7 Large-Scale Oxidations of Alcohols, Carbohydrates and Diols ................................................................................ 150

2.4 Carbohydrates .................................................................................... 151 2.4.1 Primary Alcohol Groups in Carbohydrates

to Carboxylic Acids................................................................ 152 2.4.2 Secondary Alcohol Groups in Carbohydrates

to Ketones ............................................................................... 153 2.4.3 Syntheses of Carbohydrate Natural Products/

Pharmaceuticals...................................................................... 159 2.4.4 Miscellaneous Carbohydrate Oxidations ............................... 160 2.5 Diols ................................................................................................... 160 2.5.1 Specific Examples .................................................................. 160 2.5.2 Natural Product/Pharmaceutical Syntheses

Involving Diols ....................................................................... 161 2.5.3 Desymmetrisation of Meso-Diols .......................................... 162 2.6 Miscellaneous Oxidations of Alcohols .............................................. 162References ......................................................................................................... 163

3 oxidation of alkenes, arenes and alkynes ........................................... 173 3.1 Oxidation of Alkenes Involving No C=C Bond Cleavage ................. 173 3.1.1 Epoxidation of Cyclic and Linear Alkenes ............................ 174 3.1.2 Cis-Dihydroxylation of Alkenes ............................................ 181 3.1.3 Ketohydroxylations ............................................................... 185 3.2 Oxidative Cleavage of Alkenes .......................................................... 192 3.2.1 Alkene Cleavage to Aldehydes or Ketones ............................ 192 3.2.2 Alkene Cleavage to Carboxylic Acids ................................... 196 3.3 Oxidation of Arenes ........................................................................... 200 3.3.1 Aromatic Rings to Carboxylic Acids or to CO

2 ..................... 201

3.3.2 Phenols ................................................................................... 203 3.3.3 Polycyclic Arenes ................................................................... 203 3.3.4 Oxidative Coupling of Arenes and Napththols ...................... 204 3.3.5 Large-Scale Oxidations of Arenes ......................................... 204 3.3.6 Aromatic Substrates Not Covered Here

but Included in Chapter 1 ....................................................... 204 3.4 Oxidation of Alkynes ......................................................................... 205 3.4.1 Oxidation of Alkynes Involving No

C=C Bond Cleavage ............................................................... 205 3.4.2 Oxidative Cleavage of Alkynes to Carboxylic Acids ............. 206 References ......................................................................................................... 207

4 oxidation of alkanes ................................................................................ 215 4.1 Oxidation of C–H Bonds in Alkanes ................................................. 215 4.1.1 Aldehydes and Other R1R2R3CH Substrates .......................... 215

x Contents

4.1.2 Methylene Groups to Ketones ................................................ 216 4.1.3 Methyl Groups to Aldehydes or Carboxylic Acids ................ 219 4.1.4 Cyclic Alkanes ....................................................................... 219 4.1.5 Large-Scale Oxidations of Alkanes ....................................... 222 4.1.6 Alkane Oxidations Not Covered Here but

Included in Chapter 1 ............................................................. 222 4.2 Oxidation of C–C Bonds in Alkanes .................................................. 223References ......................................................................................................... 224

5 oxidations of amines, amides, ethers, sulfides, phosphines, arsines, stibines and Miscellaneous substrates .................................... 227 5.1 Oxidation of Amines .......................................................................... 227 5.1.1 Primary Amines RCH

2NH

2 to Nitriles,

Amides or Ketones ................................................................. 227 5.1.2 Secondary Amines, R1R2NH .................................................. 229 5.1.3 Tertiary Amines R1R2R3N ...................................................... 231 5.1.4 Large-Scale Oxidation of Amines .......................................... 233 5.1.5 Amine Oxidations Not Covered Here but Included

in Chapter 1 ............................................................................ 234 5.2 Oxidation of Amides .......................................................................... 234 5.2.1 Cyclic a-Amino Acid Esters .................................................. 234 5.2.2 N-Alkyllactams; b-Lactams to Acyloxy-b-Lactams .............. 235 5.2.3 Substituted Pyrrolidines ......................................................... 235 5.2.4 Miscellaneous Amides ........................................................... 237 5.3 Oxidation of Ethers, R1R2O ................................................................ 238 5.3.1 Ethers to Esters ....................................................................... 238 5.3.2 Ethers to Lactones .................................................................. 240 5.3.3 Other Ether Oxidations .......................................................... 240 5.3.4 Ether Oxidations Not Covered Here

but Included in Chapter 1 ....................................................... 241 5.4 Oxidation of Sulfides (thioethers), R1R2S .......................................... 241 5.4.1 Sulfides to Sulfoxides ............................................................ 241 5.4.2 Sulfides to Sulfones ................................................................ 242 5.4.3 Sulfoxides to Sulfones............................................................ 243 5.4.4 Sulfides to Sulfates; Cyclic Sulfites to Sulfates ..................... 243 5.4.5 Sulfilimines to Sulfoximes; N-Sulfonylsulfilimines

to N-Sulfonylsulfoximines ..................................................... 244 5.4.6 Asymmetric Epoxidations of Sulfides ................................... 244 5.4.7 Large-Scale Oxidations of Sulfides and Sulfites .................... 244 5.4.8 Sulfide Oxidations Not Covered Here

but Included in Chapter 1 ....................................................... 245 5.5 Oxidation of Phosphines, Arsines and Stibines ................................. 245 5.5.1 Asymmetric Oxidations of Phosphines

to Phosphine Oxides ............................................................... 246

xiContents

5.6 Oxidations of Miscellaneous Substrates ............................................ 246 5.6.1 Si–H Bonds in Organosilanes ................................................ 246 5.6.2 Nitriles .................................................................................... 246 5.6.3 Hydrocarbons in Coals ........................................................... 246 5.6.4 Nitro and Halide Compounds................................................. 247 5.6.5 Azidolactones; Depyrimidination of DNA ............................ 247 5.6.6 Acids; Diones ......................................................................... 248 5.6.7 Fullerenes ............................................................................... 248References ......................................................................................................... 248

index ................................................................................................................. 253

xiii

Abbreviations which are commonly used in the book are listed here: each is defined when it first occurs in the text, but not subsequently; those used once only are not listed here. Wherever possible internationally known abbreviations (e.g. bpy, py, etc.) are used, but in some cases the usage of the authors of the papers cited have been used.

acac Acetylacetonate mono-anion (2,4-pentanedionate mono-anion)atm Atmospheres (pressure)aq. In aqueous solution, or aquated

BHT 2,6-di-tert-Butyl-4-methylphenolB.M. Bohr magnetonsBmim 1-Butyl-3-methylimidazolium mono-cationBn Benzyl, C

6H

5CH

2

Boc tert-Butoxycarbonylbpy 2,2¢-BipyridylBDTAC Benzyldimethyltetradecylammonium chlorideBTBAC Benzyltributylammonium chlorideBTEAC Benzyltriethylammonium chloride, (PhCH

2NEt

3)Cl.H

2O

Bu ButyltBu tert-ButylBz Benzoyl, C

6H

5CO

Ch. ChapterCHP Cumene hydroperoxidecinc Cinchomeronate (3,4-pyridinedicarboxylate dianion)COD Cyclo-octa-1,5-dieneCp p-C

5H

5 Mono-anion

Cp* h5-C5Me

5 Mono-anion

Cl2bpy 6,6¢-Dichloro-2,2¢-bipyridyl

Cl2pyNO 2,6-Dichloropyridine-N-oxide

DCE 1,2-DichloroethaneDDAB Didecyldimethylammonium bromideDFT Density function theoretical (calculations)

abbreviations

xiv Abbreviations

DMF Dimethylformamidedmg Dimethylglyoxime mono-aniondmp 2,9-Dimethyl-1,10-phenanthrolineDMSO, dmso Dimethylsulfoxide (upper case for solvent, lower-

for ligand)dpae 1,2-bis-(Diphenylarsino)ethanedppe 1,2-bis(Diphenylphosphino)ethanedppp 1,3-bis(Diphenylphosphino)propanedppm bis(Diphenylphosphino)methane

e Molar extinction coefficient,/ dm3 mol−1 cm−1

EDTA Ethylenediaminetetra-acetate tetra-anion EDTA4− (acid is H

4EDTA; Fig. 1.33)

e.e. Enantiomeric excessemim 1-Ethyl-3-methy l-1H-imidazolium mono-cationESR Electron spin resonanceEt EthylEtOAc Ethyl acetate

h HoursHx n-Hexyl

IR Infrared

M MolarMCPBA Metachloroperbenzoic acid or 3-chloroperbenzoic acidMe Methylmech. MechanismMe

2CO Acetone

MTCAC Methyltricaprylammonium chloridem

eff Magnetic moment in Bohr Magnetons at room temperature

(N) Nitride (N3−) ligandnapy 1, 8-Naphthyridineuas(Ru(O)

2) Asymmetric IR or Raman stretching vibration of an RuO

2 unit

us(Ru(O)2) Symmetric IR or Raman stretching vibration of an RuO

2 unit

u (Ru=(O)) Stretching vibration of an Ru=O unitnic Nicotinate dianionNMO N-methylmorpholine-N-oxiden.o. Not observed

(O) Oxo (O2−) ligandOAc AcetateOEP 2,3,7,8,12,13,17,18-Octaethylporphyrinate dianionox Oxalate, (C

2O

4)2−

Oxone® 2KHSO5. KHSO

4.K

2SO

4

Pc 4,4,¢4,¢¢¢4¢¢¢¢-Tetrasulfophthalocyaninate dianionPCB Polychlorinated biphenyls

xvAbbreviations

PDTA Propylenediamine tetra-acetate tetra-anion, PDTA4–

Ph Phenyl, C6H

5

phen 1,10-Phenanthrolinepic Picolinate (2-pyridinecarboxylate dianion)PMS Powdered (4 Å) molecular sievesPNAO Poly-N-methylmorpholine-N-oxidePPN (Ph

3P)

2N)+

Pr PropyliPr Isopropylpy Pyridinepydic Pyridine-2,6-dicarboxylic acidpyNO Pyridine N-oxidePS-TPAP Polymer-supported TPAP

R Raman spectrumRT Room temperature

SB Schiff’s baseSCE Standard calomel electrodestoich. Stoicheiometric

Tet-Me6 N, N, N¢, N¢, 3, 6-Hexamethyl-3,6-diazaoctane-1,8-diamine

TBAP Tetra-n-butylammonium perruthenate, (nBu4N)[RuO

4]

TBHP tert-Butylhydroperoxide, tBuOOHTCCA Trichloroisocyanuric acidTDCPP meso-5,10,15,20-tetrakis(2,6-Dichlorophenyl)porphyrin dianionTEMPO 2,2¢,6,6¢-Tetramethylpiperidine-N-oxyl radical (Fig. 1.40)TFPPCl

8 Octachlorotetrakis(pentafluorophenyl)porphyrinate dianion

TGA Thermal gravimetric analysisTHF TetrahydrofuranTMC Tetramethyl-tetra-azacyclotetradecane (Fig. 1.29)TMEA N,N,N¢,N¢- Tetramethyl-1,2-diaminoethanetmeda N, N, N¢, N¢-tetramethylethylenediamineTMP meso-5,10,15,20-Tetramesityl(porphyrinate) dianion (Fig. 1.24)TMPZNO Tetramethylpyrazine-N,N¢ dioxidetmtacn 1,4,7-Trimethyl-1,4,7-triazacyclononane (Fig. 1.30)tpa Tris(2-pyridylmethyl)amine)TPAP Tetra-n-propylammonium perruthenate, (nPr

4N)[RuO

4]

TPAP/NMO Oxidising mixture of TPAP and NMOTPAPORM TPAP doped on ormosil silica glassTPP meso-5,10,15,20-Tetraphenyl(porphyrin)ate dianionTroc TrichloroethoxycarbonylTTP Tetramesitylporphyrinate dianiontpy 2,2¢:6¢,2¢¢-Terpyridine

UV Ultra-violet (irradiation)

v Volts

1W.P. Griffith, Ruthenium Oxidation Complexes, Catalysis by Metal Complexes 34,DOI 10.1007/978-1-4020-9378-4_1, © Springer Science+Business Media B.V. 2011

Abstract This chapter introduces the topic and scope of the book and principally concerns the basic preparation, physical and chemical properties of Ru-based oxidation catalysts, then summarising the catalytic oxidations which they accom-plish. More detail on these is given in the succeeding four chapters. The major oxidants RuO

4 (1.2.1), perruthenate [RuO

4]− (1.3.1) – mainly TPAP, (nPr

4N)[RuO

4],

ruthenate [RuO4]2− (1.4.1), trans-Ru(O)

2(TMP) (1.4.2.5), RuCl

2(PPh

3)

3 (1.9.3) and

cis-RuCl2(dmso)

4 (1.9.4) are covered in some detail, but many other catalysts are

also discussed. In some cases brief comments are made on the mechanisms involved when data on these are given in the cited papers. There is also an Appendix (1.11) which gives brief details on the preparation of four ruthenium oxidation catalysts and selected model oxidations using them.

1.1 Overview and Introduction

This chapter is essentially a review of those ruthenium complexes which have been used as oxidation catalysts for organic substrates, emphasis being placed on such species which have been chemically well-defined and are effective catalysts. Of all the ruthenium oxidants dealt with here those which have the greatest diversity of use are RuO

4, [RuO

4]−, [RuO

4]2−, the tetramesityl porphyrinato (TMP) complex

trans-Ru(O)2(TMP), RuCl

2(PPh

3)

3, and cis-RuCl

2(dmso)

4. Many other catalysts are

covered, and the uses of two principal starting materials, RuO2.nH

2O and RuCl

3.

nH2O as precursors for a number of catalysts, discussed.

The material is arranged under the formal oxidation state of the ruthenium in the complexes, in descending order within each category, viz. Ru(VIII) to Ru(0). Individual complexes within each oxidation state are covered and for each, wherever possible, references to preparation, structural and basic physico-chemical data are given. The arrangement of complexes within each oxidation state broadly follows the sequence of donor atoms used in the book: O, N, P, As, Sb, S, C. Within each formulation ligands determining the oxidation state are placed first (e.g. (O), (N), F, Cl, Br, I, (acac) etc.), e.g. cis-RuCl

2(phen)

2) rather than cis-Ru(phen)

2Cl

2.

Chapter 1The Chemistry of Ruthenium Oxidation Complexes

2 1 The Chemistry of Ruthenium Oxidation Complexes

In general ligands are listed in order of increasing denticity within a complex, e.g. [Ru(H

2O)(bpy)(tpy)]2+ rather than [Ru(tpy)(H

2O)(bpy)]2+.

Oxidations of organic substrates by these species are then briefly considered for classes of organic substrates. Both in this and in subsequent chapters these are arranged in the order: alcohols (including carbohydrates), alkenes, arenes, alkynes, alkanes, amines, amides, ethers, sulfides, phosphines, arsines and stibines; and finally miscellaneous oxidations not covered in preceding sections. Tables of typi-cal oxidations are given in Chapters 2–5; within these tables oxidation products are arranged alphabetically in the central column. In a few cases stoicheiometric oxi-dants are covered where it is likely that such reactions might be rendered catalytic; occasionally species which show potential catalytic or stoicheiometric oxidative properties are mentioned. Electrocatalytic oxidations are covered but not heteroge-neous catalysis not patented procedures, though a few instances of supported catalysis are mentioned. Since the book concentrates on practical usage of the cata-lysts, the coverage of reaction mechanisms is deliberately light, though references are given wherever possible.

At the head of most of the oxidation sections in Chapters 2–5 a very simplified overall equation is given for the specified set of reactions, its purpose being merely to indicate the notional overall stoicheiometry of the reaction. In these [O] indicates the input of one oxygen atom, 2[O] of two, etc. from the oxidant; there being no mechanistic implications in these simplistic equations. For oxidations of a given organic substrate to the organic product the relation between a Ru starting material (assumed in this case to be an oxoruthenate) is generalised as RuNO

x with RuN+2O

x+1

(two-electron oxidation) or RuN+4Ox+2

(four-electron oxidation) as the likely catalyst or catalyst precursor. In Fig. 1.1 the example given is of RuIVO

2 and its four-electron

oxidation product RuVIIIO4.

In this and subsequent chapters the rubric:Starting Ru material/co-oxidant/solvent/temperature (if not ambient) is used.

Thus Fig. 1.1 would be written in the text as RuO2/aq. Na(IO

4), meaning that RuO

4

is generated in situ from RuO2.nH

2O and aqueous sodium periodate (only non-

ambient temperatures would appear in the rubric). Biphasic solvent mixtures with water are denoted as water-solvent.

Most of the Ru(VIII) to Ru(IV) complexes featured are oxoruthenates; although those of Ru(III), (II) – e.g. RuCl

2(PPh

3)

3 – and Ru(0) do not lie in this category,

Catalyst

Substrate

Co-oxidantOrganic oxidation

Oxidisedproduct

2[O]

e.g. 2 Na(IO3)

e.g. 2 Na(IO4)

RuVIIIO4

RuIVO2

Fig. 1.1 Simple scheme for oxoruthenate oxidation catalysis

31.1 Overview and Introduction

oxoruthenates may well be intermediates in their oxidation reactions. The modern convention of bracketing co-ordinated oxo ligands O

22− as (O) is followed except

for the homoleptic RuO4, [RuO

4]−, [RuO

4]2− species, RuO

2 and polyoxometalates.

The precursors mainly used in ruthenium oxidation catalysis are the hydrated dioxide RuO

2.nH

2O and particularly the hydrated trichloride RuCl

3.nH

2O; for

brevity throughout the text these are referred to simply as RuO2 and RuCl

3. Such

reactions are much more effective if carried out with the hydrated materials than with the anhydrous forms.

In a work of this length the use of abbreviations is essential. If an abbreviation is used in one paragraph only (as is the case for some complex ligands) it is defined in that paragraph alone.

1.1.1 Discovery of Ruthenium

Ruthenium was the last of the six platinum group metals to be isolated, and was discovered in Kazan (now capital of the Tatarstan Republic, Russian Federation) by Karl Karlovich Klaus1 (1796–1864) in 1844. The original papers were published in Russian journals which are difficult to obtain now, but were published in Western Europe in 1845 [1, 2] with a summary in English [3]. Klaus made the metal by reduc-tion of RuO

2 with H

2 and named it Ruthenium in honour of his native land (Ruthenia,

Latin for Russia); there are short biographies of him [4, 5]. Gottfried Wilhelm Osann (1797–1866) discovered what he thought was a new ele-

ment in 1827 and named it ‘Ruthenium’ but his claim is generally discounted [6, 7] though he still has some supporters [8]. Klaus was the first to make RuO

4, trans-

K2[Ru(OH)

2(O)

3] (which he regarded as K

2[RuO

4].H

2O), RuO

2 and RuCl

3, all essential

players in Ru oxidation chemistry. He assigned to ruthenium an atomic weight of 102.4 based on the Na

3[RuCl

6]/H

2 reaction [9], later raising this to 104 [10]. The modern

IUPAC recommended value for Ru is 101.07(2) [11], and its atomic number is 44.

1.1.2 Oxidation States in Ruthenium Complexes

Ruthenium and osmium are unique amongst all known metallic elements in three respects. Firstly they alone form octavalent homoleptic oxo complexes, RuO

4 and

OsO4. Secondly, complexes of these metals containing one of the eleven oxidation

states theoretically available to transition metals are known, from M(VIII) to M(-II) inclusive (M=Ru, Os), corresponding to electron configurations of d0 to d10. Good s-donors such as F−, O2− and N3− stabilise higher oxidation states while effective p-acceptors such as CO and NO+ stabilise low oxidation states. The oxo ligand O2−, the complexes of which concern us in this book, stabilises five oxidation states of Ru (VIII) to (IV) inclusive as a terminal ligand and four (VI to III) inclusive as a bridging

1 K. K. Klaus appears as C. Claus in the early German publications, which are cited here since they are more accessible than the Russian originals.

4 1 The Chemistry of Ruthenium Oxidation Complexes

ligand. It is this very wide range of oxidation states, mostly of course in conjunction with suitable co-ligands, and a generally favourable balance of kinetic lability and stability which allows so much oxidation chemistry to be carried out with ruthenium. By choosing suitable co-ligands it is possible to change the redox characteristics of the ruthenium centre and so in effect to adjust the oxidation properties of the catalyst.

1.1.3 Reviews on Ruthenium Complexes in Oxidation Reactions

There is no previous review or book of the type presented here, but there are some very useful reviews on its general chemistry. The two classic volumes by Gmelin in 1970 [12] and in 1938 [13] are invaluable for the early work, as are the magisterial bibliographies by Howe covering the period 1844–1950 [14–16]. The Seddons’ admirably comprehensive book on the element, The Chemistry of Ruthenium, covers material up to 1983 [6] and the author’s book The Chemistry of the Rarer Platinum Metals (Os, Ru, Ir and Rh) does so to 1967 [17]; less comprehensive but still useful is Simon Cotton’s Chemistry of Precious Metals [18]. There are reviews on higher [19–21] and lower [22] oxidation state complexes of the element. The Encyclopedia of Inorganic Chemistry has useful articles on its coordination and organometallic chemistry respectively [23, 24].

There is no single publication covering oxidations by ruthenium2 complexes as presented in this book. However there are some fairly general reviews, and also more specific ones which will be mentioned in the relevant parts of the text. Here we list only those which apply to Ru oxidants in general: [25–27]; biomimetic Ru oxidations [28, 29]; a book on Ruthenium in Organic Synthesis has a chapter on oxidations [30]; a review on large-scale oxidations in the phar-maceutical industry include some Ru-catalysed examples [31]. There are broad reviews on Ru complexes in organic synthesis [32]; on Ru oxo complexes as organic oxidants [33–36]; reviews on platinum-group metal oxidations including Ru [37–40]; oxidations by Ru porphyrin species [41–47] and by Ru macrocyclic complexes [48]. There are also reviews as part of more general treatments of oxidation by metal-oxo species [49–51]; on ‘green’ oxidations involving O

2 and

H2O

2 [52] and on O

2 [47, 52–54] as co-oxidants. For reviews on RuO

4 cf. 1.2.1

below and for TPAP cf. 2.1.2.

1.1.4 Reviews on Oxidations of Organic Substrates by Ru Complexes

These are included in the following chapters but are grouped together here. They include oxidations of alcohols in Ch. 2, a prime target for Ru-catalysed oxidations

2 Henceforth in most cases abbreviated as Ru.

51.1 Overview and Introduction

[19, 25–27, 29, 30, 35, 53–60]. In Ch. 3 are considered alkenes for which Ru complexes are active in epoxidation reactions [27, 30, 35, 60–62], including asymmetric epoxidations [44, 63]; cis-dihydroxylations [64–67]; ketohydroxyla-tions [28, 64, 66, 67]; alkene cleavage reactions [38, 50, 65, 68, 69, 71], arenes [27, 28], and alkynes [60, 65, 70, 71]. In Ch. 4 Ru-catalysed oxidations of alkanes are cov-ered: [19, 27, 30, 51, 72]. Finally in Ch. 5 oxidation of a series of heteroatomic substrates is considered: amines [27, 28, 30, 32, 42, 73, 74]; b-lactams and amides [27, 30]; ethers [35, 47]; sulfides [42, 46, 47]; phosphines, arsines and stibines [46, 47], and finally those few substrates which do not fall under the categories above.

1.1.5 Syntheses of Natural Products or Pharmaceuticals by Ru Catalysts

Many biologically important materials have been synthesised by using Ru catalysts as part of multi-step syntheses. The catalyst used is indicated in parentheses. These include essential steps in synthesis of the phytohormone Abscisic acid (TPAP) [75]; the marine eicosanoid Agardhilactone (TPAP) [76]; the sugar D-allose (RuO

4);

Fig. 2.13, [77]; the hormone d-Aldosterone (RuO4) [78, 79]; the antitumour styryl-

lactone (+)-Altholactone (TPAP) [80]; the macrolide Altohyrtin A (TPAP) [81]; the quassinoid (±)-Amarolide (RuO

4) [82]; the fragrance (−)-Ambrox® (RuO

4 and

[RuO4]−) (Fig. 3.20) [83]; the marine macrolide Amphidinolide T1 (TPAP) [84]; the

immunosuppressive agent Antascomicin B (TPAP) [85]; the marine natural product Antheliolide A (TPAP) [86]; the plant hormone (±)-Antheridiogen (A

An, 2) ([RuO

4]2−)

[87]; the non-proteinogenic amino acid Anticapsin (TPAP) (Fig. 2.7) [88, 89]; the cytotoxic benzolactone enamide Apicularen A (TPAP) [90]; the sugar D-Arcanose (RuO

4) [91]; the anti-malarial agent Arteether (TPAP; Fig. 2.4) [92]; the AChE

inhibitor (+)-Arisugacin A and B (TPAP) [93]; the eunicellin Astrogorgin (TPAP) [94]; the anti-parasitic Avermectin-B1a (TPAP; Fig. 2.6, 1.11) [95–97]; the antifeed-ant and growth-disruption agent Azadirachtin (TPAP) [98–100]; the alkaloid (+)-Batzelladine A (TPAP; Fig. 1.13) [101]; the antibiotic Biphenomycin B (RuO

4)

[102]; the antibiotic (−)-Borrelidin (TPAP) [103]; the neurotoxin Brevetoxin B (TPAP) [104, 105]; the limonoid Calodendrolide (TPAP) Fig. 2.8) [106]; the phero-mone R-g-Caprolactone (RuO

4) [107]; the nutritional supplement L-Carnitine (RuO

4)

[108]; the antiviral Castanospermine and 1-Epicastanospermine (TPAP) [109]; the vinca alkaloid (+)-Catharanthine (TPAP) [110]; the sesquiterpene (−)-Ceratopicanol (RuO

4, TPAP) [111]; the sugar L-Cladinose (RuO

4) [112, 113]; the antitumour agent

ent-Clavilactone B (TPAP) [114]; the synthase inhibitor (−)-CP-263,114 (TPAP) [115]; the biologically active sequiterpene (−)-Diversifolin (TPAP) [116]; the sesqui-terpene isoDrimeninol ([RuO

2Cl

3]−) [117]; the agonist Dysiherbaine (TPAP) [118]; the

marine anti-tumour agent Eleutherobin (TPAP) [119]; the analgesic (±)-Epibatidine (TPAP) [120]; the anti-growth factor 2-Epibotcinolide (TPAP) [121]; the cytotoxic (−)-7-Epicylindrospermopsin (TPAP) [122]; the carbohydrate 1-epiHyantocidin (TPAP) [123]; the alkaloid (±)-Epimaritidine (TPAP) [124]; the cytotoxic antitumour

6 1 The Chemistry of Ruthenium Oxidation Complexes

agent Epothilone C (TPAP) [125]; the antileukemic agents (−)-Eriolangin and (−)-Eriolanin (TPAP) [126]; the spirobicyclic sesquiterpene (±)-Erythrodiene (TPAP) [127]; the enzyme inhibitor (+)-Fagomine (TPAP) [128]; the cytotoxic Fasicularin (TPAP) [129]; the anti-inflammatory Flurbiprofen (RuO

4) [130]; the limonoid

Fraxinellone (TPAP) [131]; the polycyclopropane antibiotic FR-900848 (RuO4)

[132]; the plasmodial pigment Fuligorubin A (TPAP) [133]; the biologically active amino sugars Furanodictines A and B (TPAP) [134]; the antibiotic carbasugars Gabosine I and Gabosine G (TPAP) [135]; the antifungal Gambieric acids A and C (TPAP) [136]; the ether toxin Gambierol (TPAP) [137]; the growth factor Gibberellic acid ([RuO

4]2−) [138]; the anti-cancer agent (+)-Goniodiol (TPAP) [139, 140]; the

cytotoxic Gymnocin-A (TPAP) [141]; the steroidal phytohormone (22S, 23S)-28-Homobrassinolide (Fig. 3.5) (RuO

4) [142]; the acetogenin 10-Hydroxyasimicin

(TPAP [143]; the xenicane diterpene 4-Hydroxydictyolactone (TPAP) [144]; the antibiotic dl-Indolizomycin (TPAP) [145]; the carbohydrate allo-Inositol (Fig. 3.3) (RuO

4) [146]; the antitumour agent Irisquinone (TPAP) (Fig. 2.3) [147]; the alkaloid

(+)-Laccarin (RuO4) [148]; the alkaloid (±)-Lapidilectine B (TPAP) [149]; the

colony-stimulating factor Leustroducsin B (TPAP) [150]; the pheromone (±)-Lineatin (RuO

4) [151]; the antiparasitic spiroketal macrolides (+)-Milbemycin a

1 (TPAP)

[152] and (+)-Milbemycin b1 (TPAP) [153]; the cytotoxic sponge alkaloids

Motopuramines A and B (TPAP) [154]; the acetogenin Muricatetrocin C (TPAP) [155]; the sugar L-Mycarose (RuO

4) [112, 113]; the pathogenetic agent Mycocerosic

acid (RuO4) [156]; the glutamate receptor Neodysiherbaine (TPAP) [157]; the antivi-

ral nucleoside (−)-Neplanocin A (TPAP) [158]; the sesquiterpene Nortrilobolide (TPAP) [159]; the marine alkaloid Norzoanthamine (TPAP) [160]; the phosphatase inhibitor Okadaic acid (TPAP) [161]; the triterpene (+)-a-Onocerin (RuO

4) [162];

the eunicellin Ophirin B (TPAP) [94]; the antiviral (−)-Oseltamivir (RuO4) [163, 164];

the anticarcinogen Ovalicin (TPAP) [165]; the alkaloid (±)-Oxomaritidine (TPAP) [166]; the biologically active diterpene Phonactin A (TPAP) [167]; the cytotoxic agent Phorboxazole (TPAP) [168]; the macrolide Prelactone B (TPAP) [169]; the antibacterial agent Pseudomonic acid C (TPAP) [170]; the sugar D-Psicose (RuO

4)

[171]; the immunoadjuvant QS-21Aapi

(TPAP) [172]; the lipophilic maacrolide Rapamycin (TPAP) [173], cf 1.11, [174], (RuO

4) [175]; the antigenic diterpene

(+)-Resiniferatoxin (TPAP) [176]; the antitumour macrolide (+)-Rhizoxin D (TPAP) [177]; the spiroketal ionophore antibiotic Routiennocin (TPAP) [178, 179]; the metabolites Salicylihalamines A and B (TPAP) [180]; the anti-tumour stablising agents Sarcodictyins A and B (TPAP) [181]; the polypropionate Siphonarienolone (TPAP) [182]; the microbial metabolite (+)-SCH 351448 (TPAP) [183]; the antitu-mour cis-Solamin (RuO

4, TPAP) [184]; the heliobactericidal (+)-Spirolaxine methyl

ether (TPAP) [185]; the antimicrobial Squalamine (RuO4) [186]; the anticancer drug

Taxol® (TPAP) [187, 188]; the antibiotic and antiparasitic Tetronasin (TPAP) [189, 190]; the antiviral Tamiflu ((−)-Oseltamivir) (RuO

4) [163, 164]; the antitumour Tetronolide

(TPAP) [191]; the coagulation protein Thrombin (RuO4) [192]; the antibiotic

(+)-Tetronomycin (TPAP) [193]; the SERCA inhibitors Thapsigargins (TPAP) [194–196]; the sesquiterpene Thapsivillosin F and Trilobolide (TPAP) [159]; the antitumour agent Tonantzitlolone (TPAP) [197]; the sesquiterpene Trilobolide (TPAP) [159];

71.2 Ru(VIII) Complexes

the naturally occurring toxin Verrucarin A (RuO4) [198]; the therapeutic hypercholes-

terolemia agent Zaragozic acid A (TPAP) [199], and the cholesterol biosynthesis inhibitor 1233A (TPAP) [200].

1.2 Ru(VIII) Complexes

The chemistry of Ru(VIII) is dominated by that of the tetroxide, RuO4.

1.2.1 Ruthenium Tetroxide, RuO4

This was the first oxidant of Ru to be discovered and is still one of the most important and versatile. The coverage here and in subsequent chapters of organic oxidations by this reagent does not claim to be fully comprehensive but it is hoped that most of the major applications have been included. It is perhaps the most celebrated compound of Ru as an oxidant, although it does in general lack selectivity in its oxidation reactions. Its CAS number is 20427-56-9.

It is extensively used as an oxidant, mainly catalytically. There are good reviews on its oxidative properties: [12, 34–36, 39, 60, 64, 201–203]. Rylander’s historic paper [71], Courtney [60] and Gore’s [35] reviews, though early, are highly recom-mended, as is the article by Lee and van der Engh in 1973 which gives good experi-mental details for a number of organic oxidations by RuO

4 [203].

1.2.2 Preparation

Although Klaus discovered Ru in 1844 [1] it was not until 1860 that he isolated (and analysed) the volatile tetroxide, by passing Cl

2 into a solution of Na

2[RuO

4]

[10]. It is usually prepared in situ from a convenient Ru compound such as the trichloride or dioxide with a suitable oxidant, a procedure used in all but the earliest organic oxidations using RuO

4. The pure compound was made by boiling aqueous

RuCl3 with Na(BrO

3) and HCl and the vapour condensed in an ice-cooled container

[204]; from Ru(IV) or Ru(VI) species distilled with K2(S

2O

8) [205]; or from Cl

2

with aqueous K2[RuO

4] [203].

However, none of the oxidations described in this book requires the use of solid RuO

4. It is generated in solution, normally in a biphasic system from a lower

oxidation state compound such as RuO2.nH

2O or RuCl

3.nH

2O3, and a co-oxidant

replenishes the RuO4 reduced by the organic substrate (1.2.6).

3 The hydrates RuO2.nH

2O or RuCl

3.nH

2O are much more reactive than the anhydrous materials

and are always used as such for oxidation catalysis. For brevity in this book, however, they will simply be referred to as RuO

2 and RuCl

3 respectively.

8 1 The Chemistry of Ruthenium Oxidation Complexes

1.2.3 Physical Properties

These have been comprehensively reviewed: [6, 12, 17]. There is only one form of the solid [206] despite early claims that there were two. It is pale yellow and, like OsO

4, has a substantial vapour pressure at room temperatures; it melts at 25.4 ± 0.1°C

[206], boiling at 129.6 ± 0.2°C [207]. Its density is 3.28 g/cm3; the solubility in water at 0°C is 1.7% and 2.11% at 50°C, but it is very soluble in those organic solvents with which it does not react such as CCl

4 [6].

1.2.3.1 X-ray and Electron Diffraction Studies

The X-ray crystal structure of the solid shows that there are two crystal modifications, one cubic and one monoclinic, but within both forms the molecule is tetrahedral with Ru=(O) 1.695(3) Å [208]. Electron diffraction measurements on the vapour show the molecule to be tetrahedral with an Ru=(O) distance of 1.705(8) Å [209]. Similarity of the profiles of the Raman spectra of the solid, liquid and aqueous solution suggest that the molecule has tetrahedral symmetry in all three phases (Fig. 1.3) [210]. Aqueous solutions of RuO

4 are stable only at pH below 7 [211, 212].

1.2.3.2 Electronic and Vibrational Spectra

The electronic spectra of RuO4, [RuO

4]− and [RuO

4]2− in aqueous solution of the

appropriate pH are shown in Fig. 1.2 (in which the 385 and 320 nm. maxima are labelled I and II respectively) and the peaks listed in Table 1.1.

Fig. 1.2 Electronic spectra of RuO4, [RuO

4]− and [RuO

4]2− in aqueous media [6] (Reproduced

from the author and by Elsevier Ltd. With permission)

91.2 Ru(VIII) Complexes

Fig. 1.3 Raman spectra of RuO4 (a) pure liq.; (b) 5% aqueous soln.; (c) solid [210] (Reproduced

from The Royal Society of Chemistry. With permission from [210])

Table 1.1 Electronic spectraa of RuO

4, [RuO

4]− and [RuO

4]2−

in water [215]

RuO4 310 (2960)a 385 (930) –

[RuO4]− 310 (2445) 385 (2275) 460 (283)

[RuO4]2− – 385 (1030) 460 (1820)

a l in nm; e in dm3M−1cm−1

Table 1.2 Raman spectraa of RuO4 (a) pure liquid; (b) 5% aqueous

solution; (c) solid [210]

Fundamentals n1(A

1) n

2(E) n

3(F

2) n

4(F

2) Ref.

RuO4

Liquid 882a 323 914 334 [210] Aq. soln. 883 318 921 332 [210] Solid 881 324 922, 906 336,330 [210]

[RuO4]−

K[RuO4] solid 830 339 846 312 [226]

TPAP/CH2Cl

2844 (Rb) 843 (IR) 235 (IR) [475]

[RuO4]2−

[RuO4]2−/aq. KOH 807 291 828 291 [536]

a u (cm−1)b Polarised in Raman spectrum

10 1 The Chemistry of Ruthenium Oxidation Complexes

Other values of the wavelength l and molar extinction coefficient e have been given, e.g. in refs. [213] and [214] but differ little from the classic results of Connick and Hurley [215]. Such data can help in establishing which oxoruthenate species are present in oxidising solutions [212, 216]. A speciation diagram for RuO

4, [RuO

4]− and [RuO

4]2− based on electronic spectra has been given [217].

Raman spectra of RuO4 in solid, liquid and aqueous solution phases were

measured, all consistent with Td symmetry for the molecule in these environments

[210]; solid, liquid, vapour [218], solid, liquid [219], solid, liquid, vapour and for the normal and 18O-substituted form of the liquid and vapour [220]. The Raman spectrum of the pure liquid (a) is very similar in profile to that of the aqueous solu-tion (b) and of the solid (c), suggesting retention of tetrahedral symmetry in the solution (Fig. 1.3) [210]. As with electronic spectra, the Raman spectrum in par-ticular can be useful for establishing the presence of RuO

4 in catalyst solutions

[216, 221, 222].IR spectra have been reported of isotopomers of RuO

4 with 16O and 18O in argon

matrices [223]. Force-field calculations were made on the molecule [220, 224–226].

1.2.3.3 Electrochemical and Thermodynamic Data

The potential diagram for RuO4 – [RuO

4]− – [RuO

4]2− – RuO

2.aq has been given

[25], based on classic polarographic work of 1954 [227] (Fig. 1.4):Other electrochemical data on RuO

4 have been obtained [228–230]. A Pourbaix

(E-pH) diagram was given for RuO4, [Ru

4O

6]5+, [Ru

4O

6]4+, [RuO

4]−, RuO

2, Ru3+,

Ru(OH)2+ and Ru2+ [231]. Thermodynamic data on RuO4 and other Ru species were

summarised [230, 232, 233], and reviewed [234]. Static electric dipole polarisabilities of RuO

4, OsO

4 and HsO

4 were calculated [235].

1.2.4 Analysis and Toxicity

The simplest method is probably colorimetric, based on its electronic spectrum [215]. It can also be determined gravimetrically by addition of diphenylsulfide or ethanol to a solution of RuO

4; this gives RuO

2 which is then reduced to the metal

[236]. Alternatively addition of 2-propanol to a solution of RuO4 solution generates

RuO2.nH

2O [237].

Although it has been said [236] that RuO4 is less harmful to the eyes than is

OsO4, nevertheless great care should always be taken in handling it. The high

vapour pressure of the solid under ambient conditions makes it very dangerous

RuO4 [RuO4]- [RuO4]

2-0.59 V 0.2 V1.00 VRuO2.aq

Fig. 1.4 Potential diagram for RuO4-[RuO

4]−-[RuO

4]2−-RuO

2.aq [25, 227]

111.2 Ru(VIII) Complexes

to the eyes and mucous membranes. It explodes above 100°C and can also explode when mixed with a variety of substances, e.g. HI, ethanol, diethylether, ammonia and a number of organic materials [238]. There are no oxidations in this book involving solid RuO

4, although frequent reference is made to its use

in dilute solution. Nevertheless RuO4 solutions, however weak, and indeed all

Ru-containing materials, should be handled with care. Safety aspects of reactions involving RuO

4 generated from RuCl

3/TBHP/water-cyclohexane have been

examined [186].

1.2.5 RuO4 as an Organic Oxidant

As mentioned in 1.2.1 above, there are several reviews on the properties of RuO

4 as an oxidant in organic chemistry, both as a stoicheiometric but also as

a catalytic reagent [12, 34–36, 39, 60, 64, 201–203]. It is one of the most important and versatile of Ru oxidants. In the first few years after its properties in the field were realised it was often used for oxidation of alcohol groups in carbohydrates, but its versatility as an oxidant quickly became apparent and its use was extended to a variety of other reactions, notably to alkene cleavage and, more recently, to the cis-dihydroxylation and ketohydroxylation of alkenes.

The properties of RuO4 as an oxidant for organic substrates were first investi-

gated by Djerassi and Engle in 1953 [236]. The use of OsO4 as a selective oxidant

had by then been recognised, both as a stoicheiometric and as a catalytic reagent, but RuO

4 is, by virtue of its position in the Periodic Table, a much fiercer oxidant.

It was found that phenanthrene was converted to 9,10-phenanthrenequinone with a little 9,10-dihydrophenanthrene-9,10-diol, and a number of sulfides to mixtures of sulfoxides and sulfones [236]. In 1958 Berkowitz and Rylander carried out more systematic investigations using stoicheiometric RuO

4/H

2O, showing that it oxidised

primary alcohols to aldehydes or carboxylic acids, aldehydes to acids, secondary alcohols to ketones, diols to the diones; alkenes were cleaved to acids and ethers to esters, amides to imides; benzene and pyridine were oxidised to intractable prod-ucts [204]. The first use of RuO

4 as a catalyst seems to have been by Pappo and

Becker, who in 1956 generated it in situ for oxidation of cholest-4-en-3-one (1) by the unusual mixture RuO

2/Pb(OAc)

4/aq. AcOH and also effected alkyne oxidation

of 1,2-bis(1-acetoxycyclohexyl)ethyne (2) to a diketone: minimal experimental details were given. Fig. 1.5 shows two of the four oxidations accomplished by Pappo and Becker [239].

The publication used [239] is relatively obscure, and it was a paper of 1959 which really established the RuO

2/aq. Na(IO

4)/CCl

4 system4 for production of RuO

4

4 As indicated in 1.1 above, this takes the form: Ru starting material/co-oxidant/solvent; tempera-tures are only indicated if not ambient. For brevity RuO

2 and RuCl

3 denote the hydrates RuO

2.

nH2O and RuCl

3.nH

2O.

12 1 The Chemistry of Ruthenium Oxidation Complexes

[240]. Although reference is often made (e.g. [241]) to Nakata’s publication of 1963 as the standard procedure for this method, his use of it was more stoicheio-metric than catalytic; however, Nakata was one of the first to use CCl

4 as a solvent

for RuO4 [237].

1.2.6 Co-oxidants and Solvents for RuO4 Oxidations

Common procedures for making RuO4 in situ generally use hydrated RuO

2 or

RuCl3 as starting materials. The dioxide RuO

2 is said to be preferable to RuCl

3 since

oxidised chloro impurities are not formed and it may react faster than RuCl3 [242]);

hydrated rather than anhydrous RuO2 should be used [243, 244].

1.2.6.1 Co-oxidants

Sodium periodate Na(IO4) (occasionally called sodium metaperiodate) is the com-

monest co-oxidant, e.g. as RuO2/aq. Na(IO

4)/CCl

4 [240], RuO

2/aq. Na(IO

4)/EtOAc-

CH3CN [146], RuCl

3/aq. Na(IO

4)/CCl

4-CH

3CN [51]; cf. also 1.11. Very occasionally

iodide from the reduced (IO4)− can become incorporated into the organic reaction

product (Fig. 3.21) [245]. Potassium periodate, as RuCl3/aq. K(IO

4)/(BTEAC)/

CHCl3 is said to generate [RuO

4]− [246], but RuO

4 is probably the major product

[213]. The low solubility of K(IO4) in water is disadvantageous, although it has

been claimed that its use in oxidations of carbohydrates by RuO4 renders over

-oxidation less likely [247]. Periodic acid, IO(OH)5, has long been known as a

useful co-oxidant [248] and is becoming more popular, e.g. as in RuCl3/aq.

IO(OH)5/CCl

4-CH

3CN [221, 249], or RuCl

3/aq. IO(OH)

5/C

6H

12 [216]; it is however

a strong acid and this could affect some substrates. Bromate is an effective co-oxidant, e.g. RuCl

3/Na(BrO

3)/aq. HCl/CCl

4 [250], more so with ultrasound

[242, 251]. Sodium bromate is much cheaper than Na(IO4), is equally efficient for

OHO2C O

CAcO

COAc AcO

O

OOAc

(1)

(2)

Fig. 1.5 The first catalytic oxidations by RuO4 [239]